1. Field of the Invention
The present invention generally relates to a color cathode-ray tube in which a diameter of a beam spot is reduced to achieve high resolution over the entire screen. More particularly, the present invention relates to an arrangement of electrodes of an electron gun forming a main electron lens and an incorporated dividing resistor for applying an intermediate voltage to an intermediate electrode constituting the electron gun.
2. Background of the Invention
Recently, a color cathode-ray tube is more and more requested to be capable of providing an image of high resolution. One of the most decisive factor for determining the resolution is the diameter of a beam spot or a beam spot diameter formed on the screen (phosphor screen). Therefore, the beam spot of an electron gun is requested to be reduced as much as possible.
The beam spot diameter is normally expressed by the following equation (1). EQU Beam spot diameter={(M.times.d+1/2.times.M.times.C.sub.s .times..theta..sup.3).sup.2 +Rep.sup.2 }.sup.1/2 (1)
where
M: image magnification PA1 d: virtual object point diameter PA1 C.sub.s : spherical aberration coefficient PA1 .theta.: divergence angle PA1 Rep: electron repulsion
It is understood from the above equation (1) that in order to make the beam spot diameter small, it is effective to reduce the spherical aberration coefficient C.sub.s of the main focusing lens. For reducing the beam spot diameter, under the restriction imposed on designing the neck diameter, an aperture of the electron lens of the electron gun should be made as large as possible to reduce the spherical aberration upon stopping the beam.
There has been proposed a prior art such as one disclosed in Gazette of Japanese Laid-open Patent Publication No. 61-131342, for example, in which an arrangement is proposed to produce a main lens electric field with a large aperture. FIG. 1 is a diagram showing the arrangement of electrodes of an electron gun of a cathode-ray tube disclosed therein. The electron gun has a focusing electrode 105 applied with a focusing voltage Vf and an anode electrode 106 applied with an anode voltage Va in order to form a main electron lens. The focusing electrode 105 is formed of a cylindrical body 113 with an elliptical cross-section closed by an auxiliary electrode plate 114 which has three electron beam penetrating apertures 114a, 114b, 114c bored therethrough. Similarly, the anode electrode 106 is formed of a cylindrical body 115 with an elliptical cross-section closed by an auxiliary electrode 116 which has electron beam penetrating apertures 116a, 116b, 116c bored therethrough. Then, three adjacent main electric fields, which are produced between three electron beam penetrating apertures 114a, 114b, 114c and three electron beam penetrating apertures 116a, 116b, 116c, are partly overlapped. In this manner, a main lens electric field of a large aperture is produced.
Another arrangement of electrodes has been proposed in Gazette of Japanese Laid-open Patent Publication No. 8-22780, to enlarge the aperture of the main electron lens. FIG. 2 is a diagram showing such arrangement of electrodes of an electron gun of the cathode-ray tube disclosed therein. As shown in FIG. 2, the arrangement of the electrodes includes an intermediate electrode 109 formed of a cylindrical body with an elliptical cross-section interposed coaxially between the focusing electrode 105 and the anode electrode 106. The intermediate electrode 109 is applied with a potential Vm which is intermediate between the anode voltage Va and the focusing voltage Vf. Thus, the aperture of the main lens electric field is further enlarged.
In the arrangement of FIG. 2, if the intermediate electrode 109 is further elongated, then the aperture of the main lens electric field can be further enlarged. However, if the electrode is made long, the electric field effected over the focusing electrode 105, the intermediate electrode 109 and the anode electrode 106 becomes feeble. Therefore, it becomes difficult to satisfy requirements in the shape and convergence of the three beams at the same time by arranging only the auxiliary electrodes 114, 116 which close the focusing electrode 105 and the anode electrode 106, respectively. Specifically, the electric fields cannot sink deeply into the beam penetrating apertures 114a to 114c and 116a to 116c of the auxiliary electrodes 114, 116, and hence correction effect owing to the lens produced here, i.e., the correction sensitivity becomes low. For this reason, it becomes difficult to satisfy the both requirements, i.e., to keep the spot shape and convergence optimally. Accordingly, the intermediate electrode cannot be elongated unlimitedly.